Example: Brick from the Denbigh Shipwreck
Some time ago I was brought samples of brick from the wreck of a famous blockade-runner of the American Civil War, Denbigh. The swift vessel first made the run from Havana to Mobile, Alabama and later to Galveston, Texas. In 1865, while trying to reach the Confederate port at Galveston, Denbigh ran aground on a sandy shoal. When seen by the Federal flagship Fort Jackson, the gunboats Cornubia and Princess Royal started firing, and sailors from Seminole and Kennebec were sent to board and sink Denbigh. The wreck is now a protected site, and it lies in water so shallow that wreckage can be seen when the tide is especially low. The Texas Historical Commission administers the site, and the Institute of Nautical Archaeology at Texas A&M studies and preserves the wreck.
Bricks were among the supplies carried by Denbigh when it sank. A researcher brought me brick samples, still in water, recovered from the shipwreck, and he wanted to determine whether the bricks were British or French. Denbigh was a British ship but could have carried bricks from either country. To determine their country of origin, he needed to know the brick ingredients, which we could establish using the electron microprobe. Then he was planning to take that information to the Brick and Tile Museum in Somerset, England and do research there.
Above: Backscattered-electron images of the brick samples. Notice that, especially in the left
image, the clay matrix around the temper has eroded after 135 years of exposure to sea water.
Bricks, like pottery, are ceramics. Both are made of fired clay (although some bricks are instead sun-baked), and there is usually a temper, such as sand or crushed rock, added to the clay to increase resilient during drying and firing. Adobe brick, for instance, has straw added as a tempering material, and Mesopotamian mudbrick is usually about one third sand. Well-made bricks can be highly weather-resistant and be as durable as stone, but the bricks from Denbigh were more than a little worse for the wear. As I mentioned earlier, the brick samples came to me in sea water, as they had been for the past 135 years. As one can see in the backscattered-electron images above, particularly the left one, clay has eroded away in some areas from around the temper. The right image shows cracks that have developed due to the brick's initial firing, saturation in sea water for over a century, and subsequent drying.
Unfortunately, the clay matrix was so eroded and chemically altered by sea water that the measured composition was not likely representative of its original composition. The temper was mostly grains of silica (quartz), usually a few hundred microns in diameter, with a few other silicates, indicating that it was probably sand.
Above: Backscattered-electron images of magnetite inclusions within the brick samples.
Backscattered-electron imaging also revealed iron-oxide inclusions, most likely magnetite, in the brick, as shown above. The magnetite inclusions could have either be deliberately added to the clay or naturally occurring, though the later seems more likely. Magnetite can become incorporated into sand via the weathering of igneous or metamorphic rock, of which it is a common component. In some places in the world, the sand is so magnetite-rich that the beaches appear black; the western coast of New Zealand is one example. In the samples, however, the magnetite was not so abundant that they appeared black or dark. In fact, the abundance was low, just a percent or two by volume. Further analyses may have revealed more information about the parent geological material.
Armed with the chemical composition of the brick's clay (albeit altered by sea water), minerals in the temper and their sizes, and some information about the magnetite inclusions, the researcher left for the Brick and Tile Museum to search their records for the compositions of nineteenth-century French and British bricks.
I did these analyses shortly before I became the research fellow in charge of the Electron Microprobe Lab. When discussing the position with the department head at the time, I was describing a few analyses I had done recently in the lab and mentioned the brick samples from Denbigh. He turned out to be a Civil War buff, unbeknownst to me, and I will fully deny rumors that he hired me on the spot right then, although it certainly didn't hurt.
6/4/07
Added:
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
